This invention relates to a variable valve gear for an internal combustion engine, of the type which is driven by a camshaft to achieve a continuous timing and stroke adjustment of the valve.
In the known valve gear such as disclosed in German patent document DE-A 42 26 163, to change the valve timing and the course of the valve stroke, the transmission ratio of the valve lever is changed by a controlled displacement of the roller in the longitudinal direction of the valve lever. However, in the case of a normal valve stroke of approximately 8 to 10 mm, by means of the known valve stroke change, no minimal strokes of approximately 2 mm for an intake valve can be achieved, such as is necessary for effective homogenizing of the fuel-air mixture.
It is an object of the present invention to further develop a valve gear of the above mentioned type, in which a normal valve stroke can be continuously reduced to predetermined low stroke values.
This object is achieved by the variable valve gear according to the invention, in which when the roller is positioned close to the valve lever axis (by means of an adjusting lever that is controlled into a first predetermined orientation), it performs no stroke-reducing effective relative movement with respect to the valve lever. On the other hand, when the roller is in a position remote from the valve lever axis (by means of the adjusting lever being controlled into a different orientation), by virtue of its swivel movement it effects a stroke-reducing relative movement with respect to the valve lever and the control cam. The advantage of the invention is that it achieves a continuous adjustment of the valve stroke over a range of, for example, from 10 mm down to approximately 2 mm.
Since the stroke adjustment according to the invention is kinematically accompanied by a change of the cam spread, as an additional important advantage, there is an opening of an intake valve controlled to a minimal stroke, which is possible long before the top dead center of the piston is reached. By using an intake valve that is controlled, for example, in a conventional manner, in a multi-valve combustion space in conjunction with an intake valve controlled according to the invention, the taken-in fuel-air mixture is effectively homogenized, on the one hand, while the gas dynamics are high, on the other hand.
In an embodiment of the invention, a structurally simple adjusting device with low space requirements is achieved by a curved displacement path for the swivel joint, in which the curvature center point of a preferably circular-arc-shaped path is situated on an axis close to the camshaft and in parallel at least to the axis of rotation of the camshaft. By selecting the position of the curvature center point on another axis, the spread and stroke variation can be selected freely with respect to their mutual assignment.
In a further embodiment, an advantageously low space requirement is achieved by providing, between the axis of rotation of the camshaft and an axis of the valve lever, a drive shaft of the adjusting device which is parallel to these axes. This drive shaft is used to drive a control lever which extends partially around the camshaft and has a swivel joint for the adjusting lever which is arranged in the free end area; it displaces the swivel joint in a controlled manner by way of the control lever along a circular-arc-shaped path between an end position above the camshaft and an end position below the camshaft.
By means of the end position of the swivel joint which is above the camshaft, as the result of the swivel direction of the adjusting lever which is thus determined according to the invention, a stroke-reducing relative movement of the roller with respect to the respective valve lever is essentially prevented in a simple manner. By means of the end position of the swivel joint which is below the camshaft, the swivellable adjusting lever causes a stroke-reducing relative movement of the roller with respect to the valve lever and the control cam, by the displacement of the roller in the direction of the free valve lever end.
For kinematic reasons, the roller guide in or on the valve lever is designed to be concentric with respect to the axis of rotation of the camshaft, and to further reduce the stroke, toward the free valve lever end the roller guide changes into a straight line and/or into an arc which is curved in the opposite direction.
A structurally simple and stable further development of a part of the adjusting device is further achieved by arranging the drive shaft of the adjusting device to be disposed between valves of the same type on the engine side, and by non-rotatably connecting it on both sides of this bearing with respective control levers. The control levers have the two swivel joints for the adjusting levers on a common pin.
In a further embodiment, an advantageously secure guiding of the respective roller is achieved by assigning one U-shaped adjusting lever to each of the two control levers, each adjusting lever having the respective roller which is rotatably disposed between the free end areas of its legs adapted to the camshaft in a curved manner.
The invention is preferably used in a valve gear with valve levers designed as rocker levers, in which case the rocker levers are connected by hydraulic valve clearance compensating elements to the respective valve. In this case, the clearance compensating element, which should be maintained in the case of a rocker lever in the conventional arrangement, is also used for the compensation in the roller guides.
The valve gear, which is designed to be variable according to the invention, can advantageously be used in connection with two valves of the same type, which are used as intake valves for adjacent cylinders of an internal-combustion engine. In a further embodiment, this makes it possible to equip each cylinder with an additional intake valve controlled in the conventional manner. While the first intake valve (which, according to the invention is controlled to a minimal stroke) is used to homogenize the mixture and/or to provide an internal exhaust gas return, in contrast, the second intake valve (which opens later and to the full stroke) is suitable to achieve the desired filling of the combustion space. When air alone is fed by way of the second intake valve, the valve gear according to the invention can advantageously be used for lean operation of an internal-combustion engine.
Since the above-described mechanism of the variable valve gear according to the invention is switched in parallel to the main frictional connection by way of the camshaft, the roller, the valve lever and the spring-loaded valve, the forces operating in the control and adjusting lever are relatively low. Thus, a relatively low-power driving motor is sufficient for the adjusting device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.